Added a new workflow schema with examples and docs.

docs/assets/example-workflow.png

docs/assets/workflow-schema.png

docs/schema/workflows.md

examples/docs/workflows/composed.archive.yaml

examples/docs/workflows/geom_opt.archive.yaml

examples/docs/workflows/ground_state.archive.yaml

examples/docs/workflows/nested.archive.yaml

examples/docs/workflows/simple.archive.yaml

examples/docs/workflows/specialized.archive.yaml

mkdocs.yml

nomad/datamodel/datamodel.py

nomad/datamodel/metainfo/workflow2.py

tests/examples/test_docs.py